Conventional pelletizing processes generally use a plate or ring with many holes of various shapes that are used to form a pellet from a material that is forced into the holes. The material travels through the holes and exits at the other end, where it is cut to size by knives. Such extrusion of the material generally requires a large amount of force as the material drags on the entrance face and then the sides of the holes, producing some level of heating due to this work. The pelletizing process relies on some level of friction between the raw material and the die surfaces in order to compress the raw material to a higher density as it is extruded. However, excessive friction results in excessive heat, which can cause burning or oxidation of the material resulting in scrap.
One type of pelletizing operation uses a rotary extruder to mix and transport the materials to a die plate containing shaped holes that form the pellets. Another type of pelletizing operation uses a ring die that has mating rolls that force the material radially through the pelletizing holes from the inside to the outside of the ring die. As the extruded material exits the die, the strands may be cut by a knife, or set of knives, passing along the surface of the die face immediately upon exiting the die. These types of ring dies are typically cylindrical in shape with diameters ranging, for example, from about 16 inches (40.64 centimeters) to 72 inches (182.88 centimeters). The body of the die includes hundreds to thousands of holes throughout to facilitate the extrusion process. The diameters of the holes can range, for example, from about 1 mm to about 25 mm. These plate and ring extrusion dies may be used in a number of applications, such as pelletizing pet and animal feed, and wood pelletizing for bio-fuel applications.
A critical problem with these types of dies, however, is the loss of pellet quality with increasing cycles and premature mechanical failure of the die by cracking through the wall thickness in a radial orientation. While such failures could possibly be explained as the result of wear of the inner surface of the ring and the hole, failure analysis of the dies has revealed that, while wear of the inner surface of the extrusion die and the holes may occur, this is not the reason for the loss of pellet quality or the failure of the die by cracking. It has been found that the unanticipated reason for these failures is related to friction, as more fully described below.
During operation of conventional pelletizing ring extrusion dies, friction causes the temperature to increase, which causes volatile constituents in the slurry to vaporize or evaporate more quickly. This causes viscosity variations in the slurry, which in turn causes inconsistent flow and finally results in poor pellet quality. This inconsistent slurry flow causes the material to build up inside the die and increases the stress needed to extrude the slurry through the passageways. The increase in temperature and stress accelerate the fatigue crack growth in the die. The root cause of the loss of pellet quality and premature cracking of the dies is therefore mainly due to the friction at the entrance chamfer to the pelletizing holes. A decrease in the friction on the lead-in chamfer section would minimize the problems associated with the increased temperature and increase the die life and the pellet quality.
There are several approaches to control the friction of various types of surfaces. These include self-lubricating surfaces, where a liquid or solid lubricant is entrapped in the surface pores or features. Various low friction ceramic or cermet coatings may be deposited by various coating/cladding technologies. However, modifying or enhancing the surface in a manner that will not degrade the substrate properties while maintaining the low friction characteristics needed in this application is a challenge. A major problem with applying self lubricating surfaces in extrusion dies is that either the soft lubricating material will be consumed quickly by the extruding slurry, or the pores on the steel surface needed to retain the lubricant decrease the mechanical strength of the steel and can cause premature failure of the die.
While there are several coating/cladding techniques available to deposit low friction coatings, these technologies have their own problems including degradation of the substrate properties and poor dimensional control. For example, techniques such as thermal spray and plasma transfer arc do not work because the high heat input distorts the parts which then must be corrected, resulting in a high priced solution. Chemical vapor deposition (CVD) and physical vapor deposition (PVD) techniques are not considered because of the limited thickness of the state of the art technology and dimensional distortion caused by the high deposition temperatures. Traditional CVD technologies are limited to deposition temperatures greater than 800° C. Other technologies such as cladding or dip coating have also been unsuccessful as they plug the holes and/or distort the parts due to the high heat input during the process.
It would be highly desirable to provide an improved pelletizing die that demonstrates improved properties, such as lower friction on the chamfer region, with adequate wear resistance to maintain the chamfer profile and a method of manufacturing thereof.